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148gtl tx/rx light flickering

Discussion in 'General CB Services Discussion' started by groundwire, Aug 15, 2020.

  1. groundwire

    groundwire Sr. Member

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    I scored a brand new unopened in the box cobra 148gtl, turned it on and aawwwww. It dont work. Receive is perfect, when you key the mic on am or ssb the rx/tx light flickers back and forth which gives it an orange color. Am output power is fine, ssb has some output but the audio in both modes is barely there and 100% distorted.
    The only thing i found online was a bad mb3756 voltage regulator. And another guy said his was fixed by replacing tr23.
    Has anyone dealt with this problem before?


     

  2. loosecannon

    loosecannon Sr. Member

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    check TR35 which is the RX/TX switching transistor as well as the voltages on the pins of the MB3756 voltage reg. pin 6 is checked for 8 volts in RX, and pin 8 is checked in TX mode for 8 volts. my guess is that you will find an issue with the pin 8 voltage.

    if so, unsolder pin 8 from the PC board and check the pin itself in TX for the 8 volts.
    if its there and steady, then the problem is on the trace going from pin 8 to the rest of the radio.

    if the radio is from Taiwan, then it's very old and you could be having electrolytic capacitor problems so look for those on that trace. (honestly most just replace all of them).

    look for the dreaded brown goop disease and try to see if you have pins of an IC or component leads that are turning green or corroding away.
    dont just scrape the goop off, remove the parts completely and get it all.

    it is possible that the VCO just needs a tweak, and this might be evidenced by the radio working better on one end of the band then it does on the other.
    you can put a DC voltmeter on TP9 and set the radio for about 3.5 volts on channel 40.
    or you can hold the mic keyed, and just tweak L19 slightly in either direction and see if that gets the TX light to turn on all the way.

    im afraid that because of age, CB radio problems arent just due to bad modding and misuse like they used to be. Now its just a bunch of people buying and selling the same radios because they can't figure out the weird problems it has.

    When they finally get to someone like me, the solution becomes an entire overhaul of the PC board, caps and brown glue be damned.
    isn't it convenient that all the parts they put that damned glue on are the hard to get parts!
    good luck with it.
    LC
     
  3. groundwire

    groundwire Sr. Member

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    Thanks for the reply. Yes its a Taiwan radio and none of that nasty brown glue is in there, its all white glue. It was sitting in storage for so many years never even taken out of the box, i was hoping it wouldn't have an issue but yup. Sat to long :(
     
  4. loosecannon

    loosecannon Sr. Member

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    white glue?
    you mean yellow and really resembles liquid nails in texture?
    LC
     
  5. groundwire

    groundwire Sr. Member

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    Nope, the whitish clear hot glue looking stuff like they use nowadays
     
  6. loosecannon

    loosecannon Sr. Member

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    huh.
    that would be the very first taiwan 148 ive ever seen that had silicone in it.
    pics or it didn't happen! Just kidding.

    i guess maybe someone already removed it all and put the silicone in. Ive seen 29's from the factory with silicone but only after the 1990s.

    the big three places to look for the goop are the back left corner of the PLL chip (pin 1), the 7.8mhz crystal filter, (and/or the 7.345 crystal near it), and the ferrite beads near the driver and final.
    if you don't see any in those places, then you got lucky.

    how is the testing going?
    LC
     
    tecnicoloco likes this.
  7. groundwire

    groundwire Sr. Member

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    Wont have time till tomorrow morning. I hope its an easy fix, i dont want a cb store working on my gear. I see online cobra electronics will fix out of warranty cb radios. I wonder if they would touch a 148? Or come up with excuses not to
    Hahaha,omg. Just looked at it, its a Malaysia rig, my bad
     
    #7 groundwire, Aug 15, 2020
    Last edited: Aug 15, 2020
  8. Cobra999

    Cobra999 New Member

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    How did you make out? I have a 148 with an oddly similar situation. In my case if I key up and don’t talk, TX stays on, but if I shout or whistle, things start to flicker. I am waiting on a mic converter so I can use a 4 pin on it as I suspect my 5 pin has issues but wont be sure till I can switch it out..
     
    Slowmover likes this.
  9. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Ok, to help everyone involved...

    The MB3756 - if it's from older units that have the 5-pin plug, the ability to stay regulated it can handle more than 1 amp draw amongst all those pins.

    The newer units with the 4-pin plug, have a power rewire issue, they go thru a power choke with everything, for everything in the radio, requiring the use of the Power Choke and it's inherent voltage drop when current draw goes up.

    This causes the "constriction" to affect the MB3756's ability to stay regulated and provide service without dropout - so when you drive the radio with heavy voice peaks in AM or SSB, The power supply to the MB3756 will begin to drop out.

    Most times this is not a problem, but if you have a single power supply feed system that puts all the power demands thru the power supply choke, you will have these problems.

    Solution? Locate the Power Choke and break the feeder that goes to the AM Regulator - make it it's own power feed.

    Biggest clue is the power switch and power input pins to power the radio - once you locate the input to the radio, locate and find, the power choke, from the power switch, you'll see it takes in everything from the input to the power choke, then you can fix the power feed to Pin 2 on the MB3756...by giving the MB3756 it's own power from the switch - remove the jumper that goes to Pin 2 MB3756 by R244 - the Jumper closest to the MB3756 (runs parallel to it) the hole towards the rear panel is the power to Pin 2.

    You may find Cobras with two types of wiring of the power switch, one BEFORE the power choke, one AFTER the power choke - leaving the power filter in line all the time on the power rails. So be ready.

    Best way to revert back to OEM is to rewire the power switch to BEFORE the Power choke ,and you may need to run a separate feed wire to feed Pin 2 from the switched side of the power switch.

    In the 5-pin Mic Wire older UNIDEN boards, you don't have this problem - the newer 4-pin mic wire KEPC ones, are their power all routed thru the power choke.
     
    secret squirrel likes this.
  10. secret squirrel

    secret squirrel Lustrous Potentate

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    I saw the title of your post. I first thought the lights in your house flicker when you key your 148. Then I thought, tell us how you did that.
     
    Handy Andy likes this.
  11. Cobra999

    Cobra999 New Member

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    My 148 is the 5 pin mic. I suspected the MIC had issues as it was a old ARIES 509 and just moving it made a difference in the RXTX light flashing - so I got a handy 5 to 4 pin jack to use another known good mic. I just gave that a test and it still kind of sort of drops out. When I tested, the other side said I had carrier but no modulation. Given I have a 5 pin, what do you suspect in that case? I was excited to see your rewire - but I am guess that doesn’t apply to a 5 pin.

    Thx
     
    secret squirrel likes this.
  12. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Ok, knowing you have a 5-pin, and it gives the other person a silent carrier, tells me that this is more of a modulation issue. So the Mic amp itself is "current starved".

    If you've removed R131 (148GTL) - might want to put that back in - work another section to help with the dropout.

    Go back to this thread ...

    https://www.worldwidedx.com/threads/volting-a-cobra-148-gtl-final.100144/page-3#post-761237

    The post you need to review - if modulation is all you want - then start there - my response is below that one in the thread.

    I'm not telling you to volt a final - far from it, I'm trying to offer you clues as to what you can do to help with your losses.

    [​IMG]
    If you're like me, I get pissed when I look at my energy bill, wondering where all those watts actually went - were they wasted or used or stolen?

    If that is how you feel, then take charge - meaning your radios' power supply has something stealing power, it's glory - from you, when you use it and it can't keep up...

    Start cutting down the power drains...

    Heavy saturated modulation - draws down power - it has to get it from somewhere, so if it takes too much thru a constriction - the voltage drop is too great and in stock form - the mic amp uses the MB3756s' constant supply regulation rail - too many branches on the tree asking for power - the tree cannot, simply cannot, keep up.

    Start by reviewing your operation - does your power supply; linear or switching - is it able to keep afloat? Ever run a DVM in Voltage, hooked up across the output of and watching - monitoring - the bridge rectifiers' performance - in DC and AC? You might be surprised to find problems in the power supply you didn't know about by looking at both the DC realm, and the ripple side - by looking at the AC voltage too.

    Some linear power supplies can benefit from a capacitor overhaul - the addition of a large value (1,000uF or even 470uF) can help smooth the ripple - obtaining energy in storage thru the extra capacitance - to lessen the regulation and sag / draw down - that does occur in a linear power supply - the current demand places the pass transistor in linear region forcing a dropping of voltage across it generating heat - some of this can be avoided if the ripple effect was less - by increasing the reserve voltage to a certain degree - to help sustain the voltage for a longer period of time before the lag and drop occurs. Adding capacitance before and after the regulation circuit can aid you in keeping enough charge to handle the occasional drop in power due to demands.

    Volume control - a long abused part of the chain - contains your on-off or maybe thru a separate switch - but the switch part - carries the ENTIRE radio load across the terminals of itself and any long term arcing issues or neglect - will rear it's head here too.

    If you've had work done or recaps - and if the MB3756 cap hasn't been changed or they used a different value - that can cause a dropout in regulation, simply a performance issue.

    Have you checked the mA draw of the Driver and Final as of recent? Recheck if you can - higher mA draw had to arrive from somewhere, that is from the MB3756, not a simple power feed from the On / Off switch.

    The PLL section runs a Tripler - takes it's toll on parts - might want to look into power filter caps - not just the Electrolytic - the Disc caps here too. Age and power surges take their toll of power supply buss feed filtering caps - they decrease their effects (ESC) as they age and begin to look like resistors (ESR).

    If you try to make the Audio Chip "sing" a stronger tune, that too can cause the dropout - changing the AM regulator also does this - the latter technology realm has better performing transistors - so they can keep up with todays digital demands. Yes I said digital - it may be your correctly numbered part, but if it's not the STOCK or even if it is - what else are supplanting into the radio to affect the chain - does have long term effects.

    If you've changed or have had work done or haven't had it looked at in a while - better talk to the tech because the work they did is causing you to clip, or you have work that needs to be done to keep it going.
     
    secret squirrel likes this.
  13. Cobra999

    Cobra999 New Member

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    Just a quick update on my situation. I had severe rx/tx flickering when keyed up and pushing modulation. Upon further inspection, one of my caps was popped, specifically a 1000uf 25v cap near a small yellow transformer (I can't find a good PCB map to point this out but there is really one cap of this size).

    I replaced this with a cap I had on hand and instantly fixed the tx/rx flickering for me. time to clean up the radio now that is actually functioning!

    73s
     
    Handy Andy and TM86 like this.
  14. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    (y):)(y)
     
  15. nomadradio

    nomadradio Analog Retentive

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    Way cool! Congratulations.

    Not a good sign, though. I'm used to seeing those caps fail first in high-mileage radios. But if this radio was built around 1980, the age of other electrolytic caps in that radio may also catch up with them soon.

    Pretty sure Uniden's Taiwan factory made those radios between 1979 and 1989, more or less. The newest it can be is 32 years old.

    One cap we started replacing preemptively in Taiwan SSB radios is C175, a 470uf cap between the audio chip and the speaker circuit. Original part's voltage rating is 10 Volts. We use a 25-Volt part just as policy. Newer parts tend to be slimmer than originals, so the higher voltage rating part should fit just fine. The 10-Volt rating is risky to begin with.

    Reason for this is that when C175 breaks down in the normal way it can assassinate both the audio chip IC6 and the speaker, both at the same time. Got tired of seeing that pattern. Didn't like the risk that a radio we had serviced would just spontaneously break down that way. Only had to have that happen a couple of times before I declared that policy.

    If the radio shows any more bad caps it may be time for one of Klondike Mike's capacitor kits.

    And if everything else in the radio works perfectly for a long time, nobody will complain.

    73
     
    tecnicoloco and Cobra999 like this.

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